Method and system for synchronizing time information in ad hoc network

ABSTRACT

A method and a system for synchronizing time information in an ad-hoc network are disclosed. According to the present invention, provided is a method for synchronizing time information in an ad-hoc network as a method by which a plurality of nodes included in an ad-hoc network synchronize a time, the method comprising the steps of: transmitting, by a first node, time information obtained by correcting an initial time using a first self-correcting value at an nth beacon interval, wherein the first self-correcting value is a self-correcting value or a local averaging value at an (n−1)th beacon interval; and correcting, by a second node which has received the time information from the first node at the nth beacon interval, time information by calculating a second self-correcting value and a local average value, wherein the first self-correcting value is either the self-correcting value or the local averaging value calculated at the (n−1)th beacon interval, and the local averaging value of the second node is an averaging value of time information of one or more neighboring nodes of the second node.

CROSSE REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No.PCT/KR2013/004732 filed on May 30, 2013, which claims priority to KoreanApplication No. 10-2012-0122579 filed on Oct. 31, 2012. The applicationsare incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method and a system for synchronizingtime information in an ad-hoc network, and more particularly, to amethod and a system for synchronizing time information between nodes ina distributed processing manner in a multi-hop ad-hoc network.

BACKGROUND ART

An ad-hoc network is a network autonomously configured with communicablenodes without any fixed base station (BS) or access point (AP). In thead-hoc network, since most nodes support mobility, each node does notreceive a continuous energy supply but uses a battery with a limitedcapacity as an energy source. Particularly, among several main factorsthat nodes use energy, energy consumed in communication has a great dealof weight.

As a representative method for reducing energy consumed incommunication, there is a method in which when communication betweennodes is necessary, the nodes wake up in a communicable state, and onthe contrary, when nodes are in a standby state in which communicationbetween the nodes is unnecessary, the nodes enter into a sleep state,thereby reducing energy consumption.

However, in the method, it is very important that all nodes existing ina network simultaneously wake up and enter into the sleep state whilehaving the same time information, and a technique for synchronizing timebetween nodes is required.

A time synchronization function (TSF) algorithm described in the 802.11standards is used as a representative method of time synchronizationprotocols, which is proposed in a conventional art. In the TSFalgorithm, when nodes exchange time information with each other bytransmitting/receiving beacon signals between the nodes, if a beaconsignal received by a node has time information earlier than timeinformation of the node, synchronization is performed while correctingthe time information of the node to the early time information. If abeacon signal received by a node has time information later than timeinformation of the node, synchronization is not performed.

In the conventional art, its procedure is simple, and thus timesynchronization is well performed when a small number of nodes exist ina network. However, in a network having a large number of nodes, sincethe number of nodes competing in a beacon contention window increases,the opportunity that a node having early time information cantransmit/receive its own beacon signals is reduced. Hence, there is aproblem in performing time synchronization of the entire network.

SUMMARY

An embodiment of the present invention is directed to a method and asystem for synchronizing time information in an ad-hoc network, whichcan effectively perform time synchronization even in a network having alarge number of nodes.

According to an aspect of the present invention, there is provided amethod for synchronizing, by a plurality of nodes included in an ad-hocnetwork, time information in the ad-hoc network, the method including:transmitting, by a first node, time information corrected using aninitial time and a first self-correcting value at an nth beaconinterval, wherein the first self-correcting value is a local averagingvalue at a previous beacon interval; and correcting, by a second nodewhich has received the time information from the first node at the nthbeacon interval, its own time information by calculating a secondself-correcting value and a local averaging value, wherein the localaveraging value of the second node is an averaging value of timeinformation of one or more neighboring node of the second node.

When the first node receives time information from one or moreneighboring nodes at an (n+1)th beacon interval, the first node maydetermine the first self-correcting value as a self-correcting value atthe (n+1)th beacon interval, and correct its own time information byusing a local averaging value calculated using an averaging value of thetime information received from the one or more neighboring nodes and theself-correcting value at the (n+1)th beacon interval.

When the second node transmits its own time information to the one ormore neighboring nodes at the (n+1)th beacon interval, the second nodemay correct its own time information by using the local averaging valueat the nth beacon interval as the self-correcting value at the (n+1)thbeacon interval.

When an initial time of the second node is earlier than a time(s) of theone or more neighboring nodes, the second node may correct its own timeinformation by subtracting the second self-correcting value and thelocal averaging value at the nth beacon interval from the initial timeat the nth beacon interval.

When the initial time of the second node is later than the time(s) ofthe one or more neighboring nodes, the second node may correct its owntime information by adding up the second self-correcting value and thelocal averaging value at the nth beacon interval to the initial time atthe nth beacon interval.

The local averaging value of the second node may be calculated accordingto the following Equation:

$\begin{matrix}{{\Delta_{i}^{avg}(n)} = {\frac{( {( {\sum\limits_{({j \in {N{({i,n})}}})}{{\overset{\sim}{T\;}}_{j}^{sc}(n)}} ) + {{\overset{\sim}{T}}_{i}^{sc}(n)}} )}{{{N( {i,n} )}} + 1}.}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Here, N is a number of neighboring nodes, i is an identifier of thesecond node, j is an identifier of a neighboring node, and TSC is acorrected time.

According to another aspect of the present invention, there is provideda method for synchronizing, by a plurality of nodes included in anad-hoc network, time information in the ad-hoc network, the methodincluding: receiving, by a node, a beacon signal including timeinformation from one or more neighboring nodes at an nth beaconinterval; determining a local averaging value at a previous beaconinterval as a self-correcting value at the nth beacon interval;calculating a local averaging value by using an averaging value of thetime information of the one or more neighboring nodes; and correctingits own time information by using the self-correcting value, the localaveraging value, and an initial time.

According to still another aspect of the present invention, there isprovided a method for synchronizing, by a plurality of nodes included inan ad-hoc network, time information in the ad-hoc network, the methodincluding: receiving, by a node, a beacon signal including timeinformation from one or more neighboring nodes at an nth beaconinterval; calculating a self-correcting value by using a differencebetween an initial time and a corrected time at a previous beaconinterval; calculating a local averaging value by using an averagingvalue of the time information of the one or more neighboring nodes; andcorrecting its own time information by using the self-correcting value,the local averaging value, and the initial time.

According to yet another aspect of the present invention, there isprovided a computer-readable recording medium for executing the method.

According to still yet another aspect of the present invention, there isprovided a system for synchronizing time information in an ad-hocnetwork, the system including: a first node configured to transmit timeinformation corrected using an initial time and a first self-correctingvalue at an nth beacon interval, wherein the first self-correcting valueis a local averaging value at a previous beacon interval; and a secondnode configured to correct its own time information by receiving thetime information from the first node at the nth beacon interval andcalculating a second self-correcting value and a local averaging value,wherein the first self-correcting value is one of a self-correctingvalue and a local averaging value, which are calculated at an (n−1)thbeacon interval, and wherein the local averaging value of the secondnode is an averaging value of time information of one or moreneighboring nodes of the second node.

It should be understood that different embodiments of the invention,including those described under different aspects of the invention, aremeant to be generally applicable to all aspects of the invention. Anyembodiment may be combined with any other embodiment unlessinappropriate. All examples are illustrative and non-limiting.

According to the present invention, a synchronization method throughdistributed processing is used, and hence nodes do not compete in abeacon contention window. Further, a method is used in which a nodereceives time information of neighboring nodes and corrects its own timeinformation by a difference between the time information and anaveraging value of the received time information. Hence, it is possibleto efficiently synchronize time information between nodes.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating an ad-hoc network systemaccording to the present invention.

FIG. 2 is a view illustrating a configuration of a node according to anembodiment of the present invention.

FIG. 3 is a view illustrating time information of node i aftercalculation of a self-correcting value and time information of the nodei after calculation of a local averaging value at an nth beaconinterval.

FIG. 4 is a view illustrating a process of correcting time informationof a node that transmits a beacon signal including its own timeinformation.

FIG. 5 is a view a process of correcting time information of a node thatreceives a beacon signal including time information of neighboringnodes.

FIG. 6 is a view illustrating a state in which a plurality of nodes arearranged according to an embodiment of the present invention.

FIG. 7 is a view illustrating a state in which time information iscorrected at every beacon interval according to an embodiment of thepresent invention.

FIG. 8 is a flowchart illustrating a process of correcting timeinformation of a node that receives a beacon signal according to anembodiment of the present invention.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will be described belowin more detail with reference to the accompanying drawings. The presentinvention may, however, be embodied in different forms and should not beconstrued as limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the present invention tothose skilled in the art. Throughout the disclosure, like referencenumerals refer to like parts throughout the various figures andembodiments of the present invention.

FIG. 1 is a schematic view illustrating an ad-hoc network systemaccording to the present invention.

As shown in FIG. 1, the ad-hoc network according to the presentinvention may include a plurality of nodes 100-n. When the state of theplurality of nodes 100-n is changed from a sleep state to a wake-upstate, the plurality of nodes 100 n set communication linkstherebetween.

As shown in FIG. 2, the node 100 according to the present invention mayinclude a controller 200 and a communication unit 202.

When a beacon signal including time information is transmitted/receivedthrough the communication unit 202, the controller 200 corrects the timeinformation by calculating a self-correcting (SC) value or localaveraging value using the time information.

According to an exemplary embodiment of the present invention, a nodereceiving a beacon signal simultaneously performs calculation of aself-correcting value and calculation of a local averaging value, and anode transmitting a beacon signal transmits the beacon signal includingtime information to which the self-correcting value is reflected.

According to the present invention, the calculation of theself-correcting value or the calculation of the local averaging value isperformed at every beacon interval, and each node corrects timeinformation by reflecting a local averaging value calculated at aprevious beacon interval as a self-correcting value at a current pointof time.

Preferably, the calculation of a local averaging value may be performedat every interval where a corresponding node receives a beacon signal.

Hereinafter, a process of synchronizing time information according tothe present invention will be described in detail with reference toequations.

According to the present invention, a time model of each node may beexpressed as shown in Equation 1.T(t)=β*t+ε(t)+T(0)  Equation 1

Here, β is a skew time, t is a current time, ε(t) is noise with respectto time, and T(0) is an initial draft time (initial time of a clock).

The time model of each node may be expressed as shown in Equation 2 byintroducing a beacon interval into Equation 1.{tilde over (T)} _(i) ^(sc) ={tilde over (T)} _(i) ^(ini)(n)+Δ_(i)^(sc)(n)  Equation 2

Here, n is a beacon interval sequence.

As shown in Equation 3, Δ_(i)(n) is a value obtained by adding up aself-correcting value of a node using its own time information at aprevious beacon interval (nth beacon interval) and a local averagingvalue using an averaging value of time information of neighboring nodesso as to correct time information at a current point of time ((n+1)thbeacon interval).Δ_(i)(n)=Δ_(i) ^(sc)(n)+Δ_(i) ^(ave)(n)  Equation 3

Hereinafter, a process of synchronizing time information of each nodeaccording to the present invention will be described in detail withreference to drawings.

FIG. 3 is a view illustrating time information of a node i aftercalculation of a self-correcting value and time information of the nodei after calculation of a local averaging value at an nth beaconinterval. FIG. 4 is a view illustrating a process of correcting timeinformation of a node that transmits a beacon signal including its owntime information. FIG. 5 is a view a process of correcting timeinformation of a node that receives a beacon signal including timeinformation of neighboring nodes.

In FIG. 3, {tilde over (T)}_(i) ^(ini)(n) is an initial time at an nthbeacon interval of a predetermined node i. The node calculates aself-correcting value Δ_(i) ^(sc)(n) by using its own time informationat a previous beacon interval ((n−1)th beacon interval), and adds up thecalculated self-correcting value to initial time information as shown inEquation 4.{tilde over (T)} _(i) ^(sc)(n)={tilde over (T)} _(i) ^(ini)(n)+Δ_(i)^(sc)(n)  Equation 4

In order to evaluate a self-correcting value Δ_(i) ^(sc)(n) of each ofnodes transmitting a beacon signal including time information at the nthbeacon interval, the node uses a difference between the time correctedby the self-correcting value at the previous beacon interval and theinitial time as shown in Equation 5.

In order to evaluate a self-correcting value of each of nodes which havereceived the beacon signal, the node uses a difference between the timeto which both the local averaging value and the self-correcting value atthe previous beacon interval are reflected and the time to which onlythe self-correcting value is reflected as shown in Equation 6.Δ_(i) ^(sc)(n)={tilde over (T)} _(i) ^(sc)(n−1)−{tilde over (T)} _(i)^(ini)(n−1)  Equation 5Δ_(i) ^(sc)(n)={tilde over (T)} _(i) ^(avg)(n−1)−{tilde over (T)} _(i)^(sc)(n−1)  Equation 6

In the present invention, the self-correcting value of the nodetransmitting the beacon signal may be a local averaging value calculatedat the previous beacon interval (see FIG. 7).

After that, each of all the nodes transmits/receives time informationto/from another node. Each of nodes receiving signals evaluates atime-averaging value Δ_(i) ^(avg)(n) by using an averaging value of timeinformation of neighboring nodes as shown in FIG. 8, and corrects timeinformation by adding up the evaluated value as shown in Equation 7.

$\begin{matrix}{{{\overset{\sim}{T}}_{i}^{avg}(n)} = {{{\overset{\sim}{T}}_{i}^{sc}(n)} + {\Delta_{i}^{avg}(n)}}} & {{Equation}\mspace{14mu} 7} \\{{\Delta_{i}^{avg}(n)} = \frac{( {( {\sum\limits_{({j \in {N{({i,n})}}})}\;{{\overset{\sim}{T\;}}_{j}^{sc}(n)}} ) + {{\overset{\sim}{T}}_{i}^{sc}(n)}} )}{{{N( {i,n} )}} + 1}} & {{Equation}\mspace{14mu} 8}\end{matrix}$

Here, N is a number of neighboring nodes, i is an identifier of a secondnode, j is an identifier of a neighboring node, and T^(SC) is acorrected time.

By repeatedly performing the process described above, the difference intime information between the nodes is reduced as shown in FIG. 7.

FIG. 6 is a view illustrating a state in which a plurality of nodes arearranged according to an embodiment of the present invention. FIG. 7 isa view illustrating a state in which time information is corrected atevery beacon interval according to an embodiment of the presentinvention.

Referring to FIGS. 6 and 7, each of nodes A and C transmits a beaconsignal including its own time information to node B connected to thenode in the neighborhood of the at a first beacon interval.

By using the transmitted beacon signal, the node B, the node Bcalculates a local averaging value using an averaging value of timeinformation of nodes located in the neighborhood of the node B as shownin Equation 8, and corrects its own time information using thecalculated local averaging value.

Since the node B transmits a beacon signal at a second beacon interval,the node B adds up a self-correcting value at the previous beaconinterval to its own time information (an initial time at the secondbeacon interval) and then transmits the time information to the nodes Aand C connected to the node B in the neighborhood of the node B. Each ofthe nodes A and C also corrects its own time information by using aself-correcting value at the previous beacon interval and a localaveraging value using an averaging value of time information of nodesconnected to the node in the neighborhood of the node.

Similarly, at beacon intervals from a third beacon interval, a nodetransmitting time information adds up an initial time at a currentbeacon interval to its own time information by using a local averagingvalue at the current beacon interval as a self-correcting value and thentransmits the time information to nodes connected to the node at theneighborhood of the node. A node which has received the time informationalso adds up a self-correcting value to its own time information andthen calculates a local averaging value using an averaging value of timeinformation of nodes connected to the node in the neighborhood of thenode, thereby adding up the calculated local averaging value to its timeinformation.

As shown in FIG. 7, among nodes receiving/transmitting a beacon signal,the time information of a node having an early time is corrected todecrease, and the time information of a node having a late time iscorrected to increase. Thus, as the beacon interval increases, times ofa plurality of nodes converge.

FIG. 8 is a flowchart illustrating a process of correcting timeinformation of a node that receives a beacon signal according to anembodiment of the present invention.

Referring to FIG. 8, when a node receives beacon signals fromneighboring nodes (step S800), the corresponding node decides whether alocal averaging value exists at a previous beacon interval (step S802).

When the local averaging value does not exist at the previous beaconinterval, the node determines a self-correcting value at the previousbeacon interval as a self-correcting value at a current beacon interval(step S804).

For example, when the current beacon interval is an nth beacon interval,the self-correcting value at the previous beacon interval in step S804may be a self-correcting value at an (n−1)th beacon interval, which maybe a local averaging value calculated at the (n−1)th beacon interval.

Meanwhile, when it is decided in step S802 that the local averagingvalue exists at the previous beacon interval, the node determines thelocal averaging value as the self-correcting value at the current beaconinterval (step S806).

According to the present invention, the self-correcting value at thecurrent beacon interval is determined as the most lately calculatedlocal averaging value.

The node determines the self-correcting value through theabove-described process, and simultaneously calculates a local averagingvalue by using an averaging value of time information of neighboringnodes at the current beacon interval (step S808).

The node corrects its own time information by using the local averagingvalue and the self-correcting value, determined through theabove-described process, and an initial time at the current beaconinterval (step S810).

As described above, in the case of a node having an early time in thead-hoc network, a local averaging value and a self-correcting value maybe subtracted from the initial time at the current beacon interval. Inthe case of a node having a late time, a local averaging value and aself-correcting value may be added up to the initial time at the currentbeacon interval.

According to the present invention, a node transmitting a beacon signaldetermines, as a self-correcting value, a self-correcting valuecalculated using time information at a previous beacon interval or alocal averaging value at the previous beacon interval, and transmits, toneighboring nodes, time information obtained by reflecting thedetermined self-correcting value to an initial time. Each of theneighboring nodes corrects its own time information through the processshown in FIG. 8.

Embodiments of the present invention may be implemented in a programcommand form capable of being performed through various computer meansto be recorded in a computer-readable recording medium. Thecomputer-readable recording medium may include a program command, a datafile, a data structure, and the like separately or in a combinationthereof. The program command recorded in the recording medium may be acommand designed or configured specially for the present invention, orusably known to a person having ordinary skill in the computer softwareart. Examples of the computer-readable recording medium include magneticmedia such as hard disks, floppy disks, and magnetic tapes, opticalmedia such as CD-ROM and DVD, magneto-optical media such as floppydisks, and a hardware device such as ROM, RAM, and flash memory, whichis configured to store and perform program commands. Examples of theprogram commands include a machine language code made by a compiler anda high-level language code implemented using an interpreter by acomputer. The hardware device can be configured as at least one softwaremodule to perform the operation of embodiments of the present invention,and vice versa.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

The invention claimed is:
 1. A method for synchronizing, by a pluralityof nodes included in an ad-hoc network, time information in the ad-hocnetwork, the method comprising: transmitting, by a first node, timeinformation corrected using an initial time and a first self-correctingvalue at an nth beacon interval, wherein the first self-correcting valueis a local averaging value at a previous beacon interval; andcorrecting, by a second node which has received the time informationfrom the first node at the nth beacon interval, its own time informationby calculating a second self-correcting value and a local averagingvalue, wherein the local averaging value of the second node is anaveraging value of time information of one or more neighboring node ofthe second node, and the local averaging value of the second node iscalculated according to the following Equation:${{\Delta_{i}^{avg}(n)} = \frac{( {( {\sum\limits_{({j \in {N{({i,n})}}})}{{\overset{\sim}{T\;}}_{j}^{sc}(n)}} ) + {{\overset{\sim}{T}}_{i}^{sc}(n)}} )}{{{N( {i,n} )}} + 1}},$where N is a number of neighboring nodes, i is an identifier of thesecond node, j is an identifier of a neighboring node, and T^(SC) is acorrected time.
 2. The method of claim 1, wherein, when the first nodereceives time information from one or more neighboring nodes at an(n+1)th beacon interval, the first node determines the firstself-correcting value as a self-correcting value at the (n+1)th beaconinterval, and corrects its own time information by using a localaveraging value calculated using an averaging value of the timeinformation received from the one or more neighboring nodes and theself-correcting value at the (n+1)th beacon interval.
 3. The method ofclaim 1, wherein, when the second node transmits its own timeinformation to the one or more neighboring nodes at the (n+1)th beaconinterval, the second node corrects its own time information by using thelocal averaging value at the nth beacon interval as the self-correctingvalue at the (n+1)th beacon interval.
 4. The method of claim 1, wherein,when an initial time of the second node is earlier than a time(s) of theone or more neighboring nodes, the second node corrects its own timeinformation by subtracting the second self-correcting value and thelocal averaging value at the nth beacon interval from the initial timeat the nth beacon interval.
 5. The method of claim 1, wherein, when theinitial time of the second node is later than the time(s) of the one ormore neighboring nodes, the second node corrects its own timeinformation by adding up the second self-correcting value and the localaveraging value at the nth beacon interval to the initial time at thenth beacon interval.
 6. A non-transitory recorded medium by a digitalprocessing device, tangibly embodying program instructions executable bythe digital processing device to perform the method of claim
 1. 7. Amethod for synchronizing, by a plurality of nodes included in an ad-hocnetwork, time information in the ad-hoc network, the method comprising:receiving, by a first node, a beacon signal including time informationfrom one or more neighboring nodes at an nth beacon interval;determining a local averaging value at a previous beacon interval as aself-correcting value at the nth beacon interval; calculating a localaveraging value at the nth beacon interval by using an averaging valueof the time information of the one or more neighboring nodes; andcorrecting its own time information by using the self-correcting value,the local averaging value at the nth beacon interval, and an initialtime, wherein the local averaging value is calculated according to thefollowing Equation:${{\Delta_{i}^{avg}(n)} = \frac{( {( {\sum\limits_{({j \in {N{({i,n})}}})}{{\overset{\sim}{T\;}}_{j}^{sc}(n)}} ) + {{\overset{\sim}{T}}_{i}^{sc}(n)}} )}{{{N( {i,n} )}} + 1}},$where N is a number of neighboring nodes, i is an identifier of thesecond node, j is an identifier of a neighboring node, and T^(SC) is acorrected time.
 8. A method for synchronizing, by a plurality of nodesincluded in an ad-hoc network, time information in the ad-hoc network,the method comprising: receiving, by a first node, a beacon signalincluding time information from one or more neighboring nodes at an nthbeacon interval; calculating a self-correcting value by using adifference between an initial time and a corrected time at a previousbeacon interval; calculating a local averaging value by using anaveraging value of the time information of the one or more neighboringnodes; and correcting its own time information by using theself-correcting value, the local averaging value, and the initial time,wherein the local averaging value is calculated according to thefollowing Equation:${{\Delta_{i}^{avg}(n)} = \frac{( {( {\sum\limits_{({j \in {N{({i,n})}}})}{{\overset{\sim}{T\;}}_{j}^{sc}(n)}} ) + {{\overset{\sim}{T}}_{i}^{sc}(n)}} )}{{{N( {i,n} )}} + 1}},$where N is a number of neighboring nodes, i is an identifier of thesecond node, j is an identifier of a neighboring node, and T^(SC) is acorrected time.
 9. A system for synchronizing time information in anad-hoc network, the system comprising: a first node configured totransmit time information corrected using an initial time and a firstself-correcting value at an nth beacon interval, wherein the firstself-correcting value is a local averaging value at a previous beaconinterval; and a second node configured to correct its own timeinformation by receiving the time information from the first node at thenth beacon interval and calculating a second self-correcting value and alocal averaging value, wherein the first self-correcting value is one ofa self-correcting value and a local averaging value, which arecalculated at an (n−1)th beacon interval, the local averaging value ofthe second node is an averaging value of time information of one or moreneighboring nodes of the second node, and the local averaging value ofthe second node is calculated according to the following Equation:${{\Delta_{i}^{avg}(n)} = \frac{( {( {\sum\limits_{({j \in {N{({i,n})}}})}{{\overset{\sim}{T\;}}_{j}^{sc}(n)}} ) + {{\overset{\sim}{T}}_{i}^{sc}(n)}} )}{{{N( {i,n} )}} + 1}},$where N is a number of neighboring nodes, i is an identifier of thesecond node, j is an identifier of a neighboring node, and T^(SC) is acorrected time.
 10. The system of claim 9, wherein, when the first nodereceives time information from one or more neighboring nodes at an(n+1)th beacon interval, the first node determines the firstself-correcting value as a self-correcting value at the (n+1)th beaconinterval, and corrects its own time information by using a localaveraging value calculated using an averaging value of the timeinformation received from the one or more neighboring nodes and theself-correcting value at the (n+1)th beacon interval.
 11. The system ofclaim 9, wherein, when the second node transmits its own timeinformation to the one or more neighboring nodes at the (n+1)th beaconinterval, the second node correct its own time information by using thelocal averaging value at the nth beacon interval as the self-correctingvalue at the (n+1)th beacon interval.